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Message-Id: <1418784331-8465-1-git-send-email-fenghua.yu@intel.com>
Date: Tue, 16 Dec 2014 18:45:31 -0800
From: "Fenghua Yu" <fenghua.yu@...el.com>
To: "H. Peter Anvin" <hpa@...ux.intel.com>,
"Ingo Molnar" <mingo@...e.hu>,
"Thomas Gleixner" <tglx@...utronix.de>,
"Glenn Williamson" <glenn.p.williamson@...el.com>
Cc: "linux-kernel" <linux-kernel@...r.kernel.org>,
"x86" <x86@...nel.org>, "Fenghua Yu" <fenghua.yu@...el.com>
Subject: [PATCH] X86-32: Allocate 256 bytes for pgd in PAE paging
From: Fenghua Yu <fenghua.yu@...el.com>
X86 32-bit machine and kernel use PAE paging, which currently wastes about
4K of memory per process on Linux where we have to reserve an entire page to
support a single 256-byte PGD structure. It would be a very good thing if
we could eliminate that wastage.
Signed-off-by: Fenghua Yu <fenghua.yu@...el.com>
---
arch/x86/mm/pgtable.c | 30 ++++++++++++++++++++++++++++++
1 file changed, 30 insertions(+)
diff --git a/arch/x86/mm/pgtable.c b/arch/x86/mm/pgtable.c
index 6fb6927..e75f923 100644
--- a/arch/x86/mm/pgtable.c
+++ b/arch/x86/mm/pgtable.c
@@ -1,5 +1,6 @@
#include <linux/mm.h>
#include <linux/gfp.h>
+#include <linux/slab.h>
#include <asm/pgalloc.h>
#include <asm/pgtable.h>
#include <asm/tlb.h>
@@ -276,7 +277,27 @@ pgd_t *pgd_alloc(struct mm_struct *mm)
pgd_t *pgd;
pmd_t *pmds[PREALLOCATED_PMDS];
+ /*
+ * Xen paravirt assumes pgd table should be in one page. pgd in
+ * 64 bit also needs to be in one page.
+ *
+ * But PAE without Xen only needs to allocate 256 bytes for pgd.
+ *
+ * So if kernel is compiled as PAE model without Xen, we allocate
+ * 256 bytes for pgd entries to save memory space.
+ *
+ * In other cases, one page is allocated for pgd. In theory, a kernel
+ * in PAE mode not running in Xen could allocate 256 bytes for pgd
+ * as well. But that will make the allocation and free more complex
+ * but not useful in reality. To simplify the code and testing, we just
+ * allocate one page when CONFIG_XEN is enabled regardelss kernel
+ * is running in Xen or not.
+ */
+#if defined(CONFIG_X86_PAE) && !defined(CONFIG_XEN)
+ pgd = kmalloc(sizeof(pgdval_t) * PTRS_PER_PGD, PGALLOC_GFP);
+#else
pgd = (pgd_t *)__get_free_page(PGALLOC_GFP);
+#endif
if (pgd == NULL)
goto out;
@@ -306,7 +327,11 @@ pgd_t *pgd_alloc(struct mm_struct *mm)
out_free_pmds:
free_pmds(pmds);
out_free_pgd:
+#if defined(CONFIG_X86_PAE) && !defined(CONFIG_XEN)
+ kfree(pgd);
+#else
free_page((unsigned long)pgd);
+#endif
out:
return NULL;
}
@@ -316,7 +341,12 @@ void pgd_free(struct mm_struct *mm, pgd_t *pgd)
pgd_mop_up_pmds(mm, pgd);
pgd_dtor(pgd);
paravirt_pgd_free(mm, pgd);
+#if defined(CONFIG_X86_PAE) && !defined(CONFIG_XEN)
+ kfree(pgd);
+#else
free_page((unsigned long)pgd);
+#endif
+
}
/*
--
1.8.1.2
--
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